Method for producing aluminum film

ABSTRACT

Provided is a method for producing an aluminum film, which is capable of continuously mass-producing aluminum films having excellent surface smoothness and a mirror surface. A method for producing an aluminum film includes electrodepositing aluminum on a surface of a substrate in an electrolyte solution, in which the electrolyte solution contains, as components, (A) an aluminum halide, (B) at least one compound selected from the group consisting of an alkyl pyridinium halide, an alkyl imidazolium halide, and a urea compound, and (C) 1,10-phenanthroline monohydrate; in which the mixing ratio (molar ratio) of the component (A) to the component (B) is in the range of 1:1 to 3:1; and in which the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled to be in the range of 0.05 to 7.5 g/L.

TECHNICAL FIELD

The present invention relates to a method for producing an aluminum film, which is capable of producing an aluminum film having excellent surface smoothness and a mirror surface.

Aluminum has many excellent characteristics, such as electrical conductivity, corrosion resistance, lightweight properties, and non-toxicity, and is widely used for plating on metal products and the like. However, since aluminum has a high affinity for oxygen and a lower oxidation-reduction potential than hydrogen, it is difficult to perform electroplating in an aqueous solution-based plating bath.

Accordingly, as the aluminum electroplating method, a method using a molten salt bath is employed. However, the plating bath using an existing molten salt needs to be heated to high temperatures. Therefore, when an attempt is made to electroplate aluminum on a resin product, the resin is melted, and it is not possible to perform electroplating, which is a problem.

In order to overcome this problem, Japanese Unexamined Patent Application Publication No. 2012-144763 (PTL 1) describes that an aluminum plating bath that is liquid at room temperature is prepared by mixing an organic chloride salt, such as 1-ethyl-3-methylimidazolium chloride (EMIC) or 1-butylpyridinium chloride (BPC), and aluminum chloride (AlCl₃), and using the plating bath, aluminum is electroplated on the surface of a resin molded body.

In particular, the EMIC-AlCl₃-based plating solution described in PTL 1 exhibits good liquid characteristics and is very useful as an aluminum plating solution. Furthermore, PTL 1 describes that by adding 1,10-phenanthroline at a concentration of 0.25 to 7.0 g/L to the aluminum plating solution, a smooth aluminum film is formed.

As a metal porous body having a three-dimensional network structure, an aluminum porous body produced by the method described in PTL 1 is very promising, for example, in improving the capacity of the positive electrode in lithium ion batteries. Since aluminum has excellent characteristics, such as electrical conductivity, corrosion resistance, and lightweight properties, an aluminum foil having a surface coated with an active material, such as lithium cobaltate, up to now has been used as a positive electrode of lithium ion batteries. By forming the positive electrode using a porous body composed of aluminum, the surface area is increased and the inside of the aluminum porous body can also be filled with the active material. Thus, even if the thickness of the electrode is increased, the active material utilization ratio does not decrease, and the active material utilization ratio per unit area is improved, enabling improvement in the capacity of the positive electrode.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.     2012-144763

SUMMARY OF INVENTION Technical Problem

As described above, an aluminum porous body having a three-dimensional network structure is very useful, and the present inventors have conducted studies on continuous mass production of aluminum porous bodies. As a result, it has been found that although a very good aluminum porous body is obtained by the method according to PTL 1, it has been observed that, in some cases, smoothness of the aluminum film may be degraded by continuous production, which requires a need to replace the plating solution with a new one.

Accordingly, it is an object of the present invention to provide a method for producing an aluminum film, which is capable of continuously mass-producing aluminum films having excellent surface smoothness and a mirror surface.

Solution to Problem

The present inventors have performed thorough studies in order to solve the problems described above. As a result, it has been conceivable that, when an aluminum plating film is continuously formed on the surface of a resin molded body subjected to conductivity-imparting treatment, the amount of 1,10-phenanthroline, which is effective for imparting smoothness, in the plating solution decreases. The 1,10-phenanthroline has two forms: anhydride and monohydrate, and PTL 1 does not particularly describe which form is better. However, it has been common general technical knowledge to use 1,10-phenanthroline anhydride instead of 1,10-phenanthroline monohydrate because aluminum chloride (AlCl₃) included in the plating solution reacts with water to generate hydrogen chloride. The reason for this is that generation of hydrogen chloride will cause corrosion of surrounding equipment and a problem of safety for the human body due to inhalation of hydrogen chloride.

However, as a result of detailed studies by the present inventors, it has been found that 1,10-phenanthroline monohydrate is effective for imparting smoothness to the plating film.

Note that, since 1,10-phenanthroline anhydride is also partially hydrated by moisture in the air, it is difficult to obtain 1,10-phenanthroline in the form of anhydride only. Therefore, in the case where 1,10-phenanthroline anhydride is added, 1,10-phenanthroline monohydrate is also mixed in the plating solution. The reason for the decrease in the smoothness of the aluminum film when the aluminum film is continuously formed by an existing method is believed to be that monohydrate included in 1,10-phenanthroline anhydride is consumed by continuous operation and the concentration of 1,10-phenanthroline monohydrate in the plating solution is decreased. Furthermore, in the case where an aluminum film is continuously formed using a plating solution by which an aluminum film with decreased smoothness is obtained, the concentration of 1,10-phenanthroline in the plating solution does not change. Therefore, it is believed that 1,10-phenanthroline anhydride is not consumed by electrical conduction, but accumulates in the plating solution.

In order to solve the problem described above, the present invention employs the following features.

(1) A method for producing an aluminum film includes electrodepositing aluminum on a surface of a substrate in an electrolyte solution, in which the electrolyte solution contains, as components, (A) an aluminum halide, (B) at least one compound selected from the group consisting of an alkyl pyridinium halide, an alkyl imidazolium halide, and a urea compound, and (C) 1,10-phenanthroline monohydrate; in which the mixing ratio (molar ratio) of the component (A) to the component (B) is in the range of 1:1 to 3:1; and in which the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled to be in the range of 0.05 to 7.5 g/L.

By the method for producing an aluminum film according to (1), it is possible to continuously mass-produce aluminum films having excellent surface smoothness and a mirror surface.

(2) In the method for producing an aluminum film according to (1), the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled by measuring an overvoltage caused by deposition of aluminum in the electrolyte solution and by adjusting the amount of 1,10-phenanthroline monohydrate added to the electrolyte solution such that the measured value of the overvoltage is within a set range.

In the invention according to (2), since it is possible to know the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution, the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution can be easily controlled.

(3) In the method for producing an aluminum film according to (1) or (2), the component (A) is aluminum chloride and the component (B) is 1-ethyl-3-methylimidazolium chloride.

In the invention according to (3), it is possible to continuously mass-produce aluminum films having more excellent surface smoothness.

(4) In the method for producing an aluminum film according to any one of (1) to (3), the substrate is a resin molded body having a three-dimensional network structure which has been subjected to conductivity-imparting treatment.

In the invention according to (4), it is possible to continuously form an aluminum film having excellent smoothness on the surface of a resin molded body having a three-dimensional network structure. Using the resulting resin structure having a three-dimensional network structure, it is possible to obtain an aluminum porous body that can be used for a positive electrode of a lithium ion battery or the like.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a method for producing an aluminum film, which is capable of continuously mass-producing aluminum films having excellent surface smoothness and a mirror surface.

DESCRIPTION OF EMBODIMENTS

A method for producing an aluminum film according to the present invention includes electrodepositing aluminum on a surface of a substrate in an electrolyte solution, in which the electrolyte solution contains, as components, (A) an aluminum halide, (B) at least one compound selected from the group consisting of an alkyl pyridinium halide, an alkyl imidazolium halide, and a urea compound, and (C) 1,10-phenanthroline monohydrate; in which the mixing ratio (molar ratio) of the component (A) to the component (B) is in the range of 1:1 to 3:1; and in which the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled to be in the range of 0.05 to 7.5 g/L.

As described above, the electrolyte solution used in the present invention is obtained by mixing at least the component (A), the component (B), and the component (C). Each of these components will be specifically described below.

As the aluminum halide, which is the component (A), any aluminum halide that forms a molten salt at about 110° C. or lower when mixed with the component (B) can be satisfactorily used. Examples thereof include aluminum chloride (AlCl₃), aluminum bromide (AlBr₃), and aluminum iodide (AlI₃), and among these, aluminum chloride is most preferable.

As the alkyl pyridinium halide, which is the component (B), any alkyl pyridinium halide that forms a molten salt at about 110° C. or lower when mixed with the component (A) can be satisfactorily used. Examples thereof include 1-butylpyridinium chloride (BPC), 1-ethylpyridinium chloride (EPC), and 1-butyl-3-methylpyridinium chloride (BMPC), and among these, 1-butylpyridinium chloride is most preferable.

As the alkyl imidazolium halide, which is the component (B), any alkyl imidazolium halide that forms a molten salt at about 110° C. or lower when mixed with the component (A) can be satisfactorily used. Examples thereof include imidazolium chloride having alkyl groups (having 1 to 5 carbon atoms) at the 1- and 3-positions, imidazolium chloride having alkyl groups (having 1 to 5 carbon atoms) at the 1-, 2-, and 3-positions, and imidazolium iodide having alkyl groups (having 1 to 5 carbon atoms) at the 1- and 3-positions. More specifically, examples thereof include 1-ethyl-3-methylimidazolium chloride (EMIC), 1-butyl-3-methylimidazolium chloride (BMIC), and 1-methyl-3-propylimidazolium chloride (MPIC). Among these, 1-ethyl-3-methylimidazolium chloride (EMIC) may be most preferably used.

The urea compound, which is the component (B), means urea or a derivative thereof, and any urea compound that forms a molten salt at about 110° C. or lower when mixed with the component (A) can be satisfactorily used.

For example, a compound represented by the formula (1) below may be preferably used.

In the formula (1), R is a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a phenyl group and two Rs may be the same or different.

As the urea compound, among these, urea or dimethylurea can be particularly preferably used.

By setting the mixing ratio (molar ratio) of the component (A) to the component (B) to be in the range of 1:1 to 3:1 in the electrolyte solution, it is possible to obtain an electrolyte solution that is suitable for electrodepositing an aluminum film on the surface of the substrate.

In the case where the molar ratio of the component (B) is assumed as 1 and when the molar ratio of the compound (A) is less than 1, electrodeposition reaction of aluminum does not occur. Furthermore, in the case where the molar ratio of the component (B) is assumed as 1 and when the molar ratio of the compound (A) is more than 3, aluminum chloride is precipitated in the electrolyte solution and incorporated into the aluminum film, resulting in degradation in the quality of the film.

Since 1,10-phenanthroline monohydrate, which is the component (C), is included in the electrolyte solution, it is possible to smooth the surface of an aluminum film formed on the surface of the substrate such that the surface of the aluminum film is in the mirror surface state.

In the present invention, the expression “the surface of the aluminum film is in the mirror surface state” refers to that the arithmetic mean roughness Ra of the surface of the aluminum film measured using a laser microscope is 0.10 μm or less.

By setting the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution to be 0.05 g/L or more, it is possible to obtain an aluminum film which has excellent smoothness and is in the mirror surface state. As the smoothness increases, stress remaining in the aluminum film increases, and adhesion between the aluminum film and the substrate decreases, or cracks occur in the aluminum film. Therefore, the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution is set to be 0.05 to 7.5 g/L.

When the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution is within the range described above, an aluminum film having excellent smoothness can be obtained. Preferably, an optimum concentration range is selected depending on the type of substrate. For example, in the case where the substrate is a copper (Cu) plate, the concentration range is preferably set to be 0.1 to 2.0 g/L. Furthermore, in the case where the substrate is a resin molded body having a three-dimensional network structure, by setting the concentration range to be 0.1 to 2.0 g/L, it is possible to obtain an aluminum film having good appearance and mechanical properties. More preferably, the concentration range is set to be 0.3 to 1.0 g/L.

1,10-Phenanthroline monohydrate is incorporated into aluminum when aluminum is electrodeposited on the surface of the substrate, and thus the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution decreases as the operation proceeds. Therefore, it is necessary to control the concentration to be within the range described above by appropriately adding 1,10-phenanthroline monohydrate to the electrolyte solution.

As the method for controlling the concentration of 1,10-phenanthroline monohydrate, it is preferable to employ a method in which the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled by measuring an overvoltage in the electrolyte solution when aluminum is deposited and by adjusting the amount of 1,10-phenanthroline monohydrate added to the electrolyte solution such that the measured value of the overvoltage is within a set range. The concentration of 1,10-phenanthroline monohydrate in the electrolyte solution correlates with the overvoltage caused by the aluminum deposition reaction. Thus, although indirectly, by adjusting the amount of 1,10-phenanthroline monohydrate added to the electrolyte solution such that the measured overvoltage is within the predetermined range, it is possible to control the concentration of 1,10-phenanthroline monohydrate in the electrolyte solution.

The set range of the overvoltage may be appropriately determined depending on the composition of the electrolyte solution. For example, in the case where the electrolyte solution is composed of aluminum chloride, 1-ethyl-3-methylimidazolium chloride, and 1,10-phenanthroline monohydrate, the overvoltage may be set to be 105 to 170 mV. Furthermore, in the case where the electrolyte solution is composed of aluminum chloride, dimethylurea, and 1,10-phenanthroline monohydrate, the overvoltage may be set to be 120 to 180 mV.

The measurement of the overvoltage may be performed continuously or periodically with an interval between successive measurements. Furthermore, at the time of measurement of the overvoltage, the electrolyte solution may be taken out from the system and measured, or measurement may be performed by providing electrodes in the electrolyte solution in the plating tank in which the aluminum film is produced.

The term “overvoltage” refers to the absolute value of the difference between the theoretical deposition potential of aluminum and the potential at which deposition of aluminum actually starts. In order to measure the overvoltage, first, an anode and a cathode are provided in the electrolyte solution, a voltage is applied between the two, and the potential at which aluminum starts to deposit, i.e., the voltage at which current starts to flow, is measured. The potential difference between the potential at that time and the theoretical potential (equilibrium electrode potential) is calculated as the overvoltage. As the anode, aluminum may be used, and as the cathode, for example, platinum, glassy carbon, or the like may be used.

1,10-Phenanthroline has two forms: monohydrate and anhydride. In the present invention, the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled to be in the range of 0.05 to 7.5 g/L. As long as the concentration of 1,10-phenanthroline monohydrate is within the set range, the electrolyte solution may contain 1,10-phenanthroline anhydride. In this case, the ratio of 1,10-phenanthroline monohydrate relative to the total amount of 1,10-phenanthroline monohydrate and 1,10-phenanthroline anhydride is preferably set to be 1% to 100% by mass, more preferably 10% to 60% by mass, and still more preferably 20% to 30% by mass.

The electrolyte solution may contain an addition agent and the like in addition to the component (A), the component (B), and the component (C). For example, when the electrolyte solution contains, as a brightener, at least one selected from the group consisting of an organic solvent, a nitrogen-containing heterocyclic compound, and a sulfur-containing heterocyclic compound, it is possible to enhance the surface glossiness of the aluminum film, which is preferable. In this case, the concentration of the brightener in the electrolyte solution is preferably set to be 0.01 to 10.0 g/L, more preferably 0.5 to 7.5 g/L, and still more preferably 2.5 to 5.0 g/L.

As the organic solvent, for example, benzene, xylene, toluene, tetralin, or the like can be preferably used.

As the nitrogen-containing heterocyclic compound, a compound having 3 to 14 carbon atoms is preferable. For example, benzotriazole, pyridine, pyrazine, bipyridine, or the like can be preferably used.

As the sulfur-containing heterocyclic compound, for example, thiourea, ethylene thiourea, phenothiazine, or the like can be preferably used.

In the method for producing an aluminum film according to the present invention, preferably, aluminum is electrodeposited on the surface of the substrate while controlling the temperature of the electrolyte solution to be 15° C. to 110° C. By setting the temperature of the electrolyte solution at 15° C. or higher, the viscosity of the electrolyte solution can be sufficiently decreased, and the electrodeposition efficiency of aluminum can be improved. Furthermore, by setting the temperature of the electrolyte solution at 110° C. or lower, volatilization of the aluminum halide can be suppressed. The temperature of the electrolyte solution is more preferably 30° C. to 60° C., and still more preferably 40° C. to 50° C.

In the method for producing an aluminum film according to the present invention, in order to electrodeposit aluminum on the surface of a substrate in the electrolyte solution, an aluminum electrode (anode) is provided in the electrolyte solution and electrically connected to the substrate in the electrolyte solution such that the substrate serves as a cathode, and a current is applied.

In this case, preferably, aluminum is electrodeposited on the surface of the substrate at a current density of 2.0 to 10.0 A/dm². When the current density is within this range, it is possible to obtain an aluminum film having more excellent smoothness. The current density is more preferably 2.0 to 6.0 A/dm², and still more preferably 2.5 to 4.0 A/dm².

Furthermore, during electrodepositing aluminum on the surface of the substrate, the electrolyte solution may or may not be stirred.

The substrate is not particularly limited as long as it needs to have an aluminum film on the surface thereof. As the substrate, for example, a copper plate, a steel strip, a copper wire, a steel wire, a resin subjected to conductivity-imparting treatment, or the like can be used. As the resin subjected to conductivity-imparting treatment, for example, polyurethane, a melamine resin, polypropylene, polyethylene, or the like which has been subjected to conductivity-imparting treatment can be used.

The resin serving as the substrate may have any shape. By using a resin molded body having a three-dimensional network structure, it is possible to eventually produce an aluminum porous body having a three-dimensional network structure that exhibits excellent characteristics for use in various filters, catalyst carriers, electrodes for batteries, and the like, which is preferable. Furthermore, by using a resin having a nonwoven fabric shape, it is also possible to eventually produce an aluminum porous body having a porous structure. The aluminum porous body having a nonwoven fabric shape thus produced can be suitably used for various filters, catalyst carriers, electrodes for batteries, and the like.

As the resin molded body having the three-dimensional network structure, for example, a foamed resin molded body produced using polyurethane, a melamine resin, or the like can be used. Although expressed as the foamed resin molded body, a resin molded body having any shape can be selected as long as it has pores connecting with each other (interconnecting pores). For example, a body having a nonwoven fabric-like shape in which resin fibers of polypropylene, polyethylene, or the like are entangled with each other can be used instead of the foamed resin molded body.

In the following description, the porous body having a three-dimensional network structure may also be simply described as the “porous body”.

Preferably, the porous body has a porosity of 80% to 98% and a pore size of 50 to 500 μm. A polyurethane foam or foamed melamine resin has a high porosity, an interconnecting property of pores, and excellent heat decomposability, and therefore can be suitably used as a foamed resin molded body. A polyurethane foam is preferable in terms of uniformity of pores, easy availability, and the like, and a foamed melamine resin is preferable from the standpoint that a foamed resin molded body having a small pore size can be obtained. In many cases, a foamed resin molded body, such as a polyurethane foam or foamed melamine resin, has residues, such as a foaming agent and unreacted monomers, in the foam production process, and it is preferable to carry out cleaning treatment.

The porosity of the porous body is defined by the following formula:

Porosity=(1−(weight of porous material [g]/(volume of porous material [cm³]×material density)))×100 [%]

Furthermore, the pore size is determined by a method in which a magnified surface of a resin molded body is obtained by a photomicroscope or the like, the number of pores per inch (25.4 mm) is calculated as the number of cells, and an average value is obtained by the formula: average pore size=25.4 mm/number of cells.

In the present invention, the resin molded body having a three-dimensional network structure is subjected to conductivity-imparting treatment before use. Regarding the conductivity-imparting treatment on the surface of the resin, any method, including any known method, may be selected. A method in which a metal layer of nickel or the like is formed by electroless plating or a vapor phase method, or a method in which a metal or carbon layer is formed by application of a conductive coating material can be used.

By forming a metal layer on the surface of the resin by electroless plating or a vapor phase method, the conductivity of the surface of the resin can be increased. On the other hand, in the method in which conductivity is imparted to the surface of the resin by application of carbon, although which is slightly inferior, an aluminum structure produced after forming an aluminum film can be obtained without mixing a metal other than aluminum thereinto. Therefore, it is possible to produce a structure substantially composed of aluminum alone as a metal. It is also advantageous from the standpoint that conductivity can be imparted inexpensively.

In the case where conductivity-imparting treatment is performed by application of carbon, first, a carbon coating material as a conductive coating material is prepared. A suspension as the carbon coating material preferably contains, in addition to carbon particles, a binder, a dispersant, and a dispersing medium.

In the case where the resin molded body having a three-dimensional network structure is used, in order to perform application of carbon particles uniformly in the porous body, the suspension needs to maintain a uniformly suspended state. For that purpose, the suspension is preferably maintained at 20° C. to 40° C. By maintaining the temperature of the suspension at 20° C. or higher, it is possible to maintain a uniformly suspended state, it is possible to prevent the state in which only the binder is concentrated on the surface of the skeleton constituting the network structure of the porous body, and carbon particles can be applied uniformly. The layer of carbon particles uniformly applied in such a manner is unlikely to be peeled off, and it is possible to form firmly adhered metal plating. On the other hand, when the temperature of the suspension is 40° C. or lower, evaporation of the dispersant can be suppressed, and therefore, the suspension becomes unlikely to be concentrated as the application treatment time passes.

Furthermore, the particle size of carbon particles is 0.01 to 5 μm, and preferably 0.01 to 0.5 μm. When the particle size is large, particles may clog pores of the porous resin molded body or block smooth plating. When the particle size is excessively small, it is difficult to secure sufficient electrical conductivity.

EXAMPLES

The present invention will be described in more detail below on the basis of examples. However, the examples are merely illustrative and the metal porous body of the present invention is not limited thereto. It is intended that the scope of the present invention is determined by appended claims, and includes all variations of the equivalent meanings and ranges to the claims.

Example 1 Electrolyte Solution

A molten salt was prepared by mixing aluminum chloride (AlCl₃) and 1-ethyl-3-methylimidazolium chloride (EMIC) at a mixing ratio (molar ratio) of 2:1. 1,10-Phenanthroline monohydrate was added to the molten salt at a concentration of 3.0 g/L to thereby obtain an electrolyte solution.

(Formation of Aluminum Film)

Using the electrolyte solution prepared as described above, an aluminum film was electrodeposited on a surface of a substrate.

A copper (Cu) plate (20 mm×40 mm×1 mm) was used as the substrate. The substrate was connected to the negative side of a rectifier, and an aluminum plate (purity 99.99%) as a counter electrode was connected to the positive side. The temperature of the electrolyte solutions was set to be 45° C., and the current density was controlled to be 3.0 A/dm².

(Measurement of Overvoltage and Control of Concentration of 1,10-Phenanthroline Monohydrate)

An aluminum electrode (anode) and a platinum electrode (cathode) were provided in the electrolyte solution, and the overvoltage was measured. The concentration of 1,10-phenanthroline monohydrate was controlled by appropriately adding 1,10-phenanthroline monohydrate to the electrolyte solution such that the overvoltage is in the range of 105 to 170 mV.

<Evaluation of Aluminum Film>

When an aluminum film with a thickness of 20 μm was formed on the surface of the copper plate serving as the substrate, the copper plate was replaced with a new one. Using the same electrolyte solution, an aluminum film was formed on the new copper plate by the same procedure. This operation was repeated.

After repeating the operation, the arithmetic mean roughness Ra of the surface of the aluminum film formed on the 50th copper plate was measured using a laser microscope. The measured value was 0.055 μm, which confirmed a very good mirror surface state.

Example 2

An aluminum film was formed as in Example 1 except that a resin molded body having a three-dimensional network structure subjected to conductivity-imparting treatment was used as the substrate. As the resin molded body, a polyurethane foam (100 mm×30 mm rectangle) with a thickness of 1 mm and a porosity of 95%, in which the number of pores (number of cells) per inch was about 50, was used. Conductivity-imparting treatment was performed by immersing the polyurethane foam in a carbon suspension, followed by drying. The carbon suspension contained, as components, 25% of graphite and carbon black, and also contained a resin binder, a penetrating agent, and an anti-foaming agent. The particle size of carbon black was set to be 0.5 μm.

<Evaluation of Aluminum Film>

When an aluminum film with a thickness of 20 μm was formed on the surface of the polyurethane foam, serving as the substrate, subjected to conductivity-imparting treatment, the substrate was replaced with a new polyurethane foam. An aluminum film was formed on the new substrate by the same procedure. This operation was repeated.

After repeating the operation, the arithmetic mean roughness Ra of the surface of the aluminum film formed on the 50th polyurethane foam was measured using a laser microscope. The measured value was 0.10 μm, which confirmed a very good mirror surface state.

Comparative Example

An aluminum film was formed on the surface of a copper plate as in Example 1 except that 1,10-phenanthroline anhydride was used instead of the 1,10-phenanthroline monohydrate.

<Evaluation of Aluminum Film>

As in Example 1, the arithmetic mean roughness Ra of the surface of the aluminum film formed on the surface of the 50th copper plate was measured using a laser microscope. The measured value was 0.75 μm, which confirmed that surface smoothness was not good. 

1. A method for producing an aluminum film comprising electrodepositing aluminum on a surface of a substrate in an electrolyte solution, wherein the electrolyte solution contains, as components: (A) an aluminum halide, (B) at least one compound selected from the group consisting of an alkyl pyridinium halide, an alkyl imidazolium halide, and a urea compound, and (C) 1,10-phenanthroline monohydrate; wherein the mixing ratio (molar ratio) of the component (A) to the component (B) is in the range of 1:1 to 3:1; and wherein the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled to be in the range of 0.05 to 7.5 g/L.
 2. The method for producing an aluminum film according to claim 1, wherein the concentration of the 1,10-phenanthroline monohydrate in the electrolyte solution is controlled by measuring an overvoltage during electrodeposition of aluminum in the electrolyte solution and by adjusting the amount of 1,10-phenanthroline monohydrate added to the electrolyte solution such that the measured value of the overvoltage is within a set range.
 3. The method for producing an aluminum film according to claim 1, wherein the component (A) is aluminum chloride and the component (B) is 1-ethyl-3-methylimidazolium chloride.
 4. The method for producing an aluminum film according to claim 1, wherein the substrate is a resin molded body having a three-dimensional network structure which has been subjected to conductivity-imparting treatment. 